Ink jet printing involves ejecting ink droplets from orifices in a printhead onto an image receiving member to form an ink image. Inkjet printers commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from the inkjets in the printhead directly onto the final substrate. In an offset printing system, the printhead jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred to the substrate before being fused or fixed to the substrate.
Alignment among multiple printheads may be expressed as the position of one printhead relative to the image receiving member, such as a media substrate or intermediate transfer surface, or another printhead within a coordinate system of multiple axes. For purposes of discussion, the terms “cross-process direction” and “X-axis direction” refer to a direction or axis perpendicular to the direction of travel of an image receiving member past a printhead within the plane of the image receiving member. The terms “process direction” and “Y-axis direction” refer to a direction or axis parallel to the direction of an the image receiving member, the term “Z-axis” refers to an axis perpendicular to the X-Y axis plane.
One particular type of alignment parameter is printhead roll. As used herein, printhead roll refers to clockwise or counterclockwise rotation of a printhead about an axis normal to the image receiving member, i.e., the Z-axis. Printhead roll may result from mechanical vibrations and other sources of disturbances on the machine components that may alter printhead positions and/or angles with respect to the image receiving member. As a result of roll, the rows of nozzles may be arranged diagonally with respect to the process direction movement of the image receiving member. This roll may cause horizontal lines, image edges, and the like to be skewed relative to the image receiving member. If the printer controls for this skew using timing adjustments, roll can increase the magnitude of the adjustments required, potentially causing the system to run out of actuation latitude. Depending upon the arrangement of nozzles in the printhead, roll error may also produce cross-process direction uniformity defects in image areas of uniform ink density.
Various methods are known to measure printhead roll and to calibrate the printhead to reduce or eliminate the effects of printhead roll on images generated by the printhead. The known methods include printing selected marks or test patterns onto the image receiving member from the printhead to identify printhead roll. In some imaging systems, the image receiving member moves in the cross-process direction while the printhead generates the test pattern. Even comparatively small movements in the image receiving member can result in errors in printed test patterns that reduce the effectiveness of known methods for detecting printhead roll. Thus, improvements to printhead measurement and calibration procedures for detecting printhead roll are desirable.